How do rivers and rain help supply sand for inland desert dunes?

Inland desert dunes are primarily supplied with sand through the erosion of sandstones and other rocks by rainwash and stream runoff. Although these processes are spasmodic in desert environments, they are often violent, effectively breaking down rocks and transporting the resulting sediment into great interior basins.1

In these arid regions, ephemeral streams—often called arroyos—play a critical role in this transport. During irregular or seasonal rainstorms, these streams experience flash flooding, which facilitates the rapid erosion and movement of sediment. As the water flows, it carries these eroded materials downstream until the water diminishes through infiltration into the dry alluvium. Once deposited in these basins, the alluvial material becomes the primary source of sand that is subsequently picked up and shaped by the wind into sand dunes.231